The present invention relates to a film forming apparatus to be specifically used for filling fine pores of a substrate in the process of manufacturing semiconductors and other electronic devices.
The technology of filling a pore of a thin film with a small piece of metal is taking an ever-increasingly important role in the process of manufacturing semiconductors and other electronic devices. Known techniques that have been used for filling fine pores include sputtering and CVD, of which CVD is particularly suited for filling fine pores of a substrate but a notable disadvantage of this technique is that it cannot be successfully used for metals that would not be gasified. CVD is a technique for forming a film on a substrate by deposition, utilizing chemical reaction of gas, and it operates effectively for filling fine pores. However, preparation of a gaseous material that is chemically active and capable of depositing on a substrate through chemical reaction is prerequisite to the use of CVD. When, for example, causing a TiN film to grow on a Si substrate, it is necessary to form an intermediary Ti film to produce a three-layered structure of TiN/Ti/Si in order to achieve stable and low contact resistivity. However, no gas sources that allow a Ti film to glow have so far been discovered. Thus, preparation of such three-layered structures is currently conducted by sputtering.
With the technique of sputtering, a target made of a material to be used for forming a thin film and a substrate are oppositely arranged in a vacuum chamber, which is supplied with discharge gas and held to a certain reduced internal pressure level, and a negative voltage is applied to the target for electric discharge so that the motion of ionized gas molecules or ions may be accelerated by the negative voltage and they may eventually strike the target and drive out atoms on the surface of the target. Then, while the driven out atoms fly away in different directions according to the cosine rule, some of them will deposit on the substrate to become a thin film.
FIG. 1-A of the accompanying drawings schematically illustrates how atoms driven out of the target get into fine pores 2 of a substrate 1 along various directions as indicated by arrow 3 and settle there in the process of film formation using sputtering. While target atoms obliquely hitting the substrate 1 settle near the opening of each pore, practically no atoms can get to the bottom of the pore as seen from FIG. 1-B. Therefore, if the substrate 1 is heated to cause the formed film to reflow and cover the pore 2, the pore 2 may contain a void space in it as shown in FIG. 1-C. If the substrate 1 is subjected to a biased negative voltage, the formation of a void space in pores may be avoidable as FIG. 1-D shows. All in all, the conventional sputtering technique is necessarily accompanied by the problem of producing void spaces in the substrate.
In a known sputtering apparatus, the distance separating the target and the substrate is normally several centimeters or smaller than the diameter of the substrate, although it may vary depending on the size of the target to be used for sputtering. Consequently, most atoms are made to obliquely hit the substrate to give rise to the above identified problem of producing void spaces.
Referring now to FIG. 2-A of the accompanying drawings, of target atoms obliquely striking the substrate 1, those that are driven out of an end of the target 4 and strike the substrate 1 at the other end of the diagonal line connecting those two ends will be most responsible for the formation of void spaces because the angle of incidence .theta. becomes smallest for such target atoms. As shown in FIG. 2-B, some of the atoms that hit on the substrate 1 with an angle of incidence .theta. may reach into the pores 2 of the substrate 1 and those that reach into the pores 2 may predominantly stick to the lateral walls of the pores 2 if the ratio of the depth b to the diameter a of each pore 2, or b/a, and the angle of incidence show a relationship of b/a&gt;tan .theta.. In other words, the relationship of b/a.ltorsim.tan .theta. should hold true in order for a part of the atoms jumping into the pores 2 to be able to successfully reach the bottom.
Some target atoms driven out of the target 4 may collide with gas molecules to change their respective courses before they arrive at the substrate 1. Conventional sputtering apparatuses normally keep the internal gas pressure of the vacuum chamber not lower than 1.times.10.sup.-1 Pa for sputtering operation and, under this condition, each of atoms has mean free path of several centimeters which is substantially equal to the distance separating the target and the substrate in the apparatus. This means that an atom emitted from the target 4 may or may not collide with other atom before it gets to the target.
If the mean free path of atoms is smaller than the distance between the target and the substrate, each atom may collide with a plurality of gas molecules before reaching the substrate so that atoms may be dispersed in the vacuum chamber to significantly reduce the rate of forming a film.
In an attempt to solve this problem and effectively fill fine pores of a substrate, U.S. Pat. No. 4824,544 discloses a film forming apparatus in which a filter having a plurality of elongated small bores is arranged between the target and the substrate so as to allow only those gas molecules that are approaching the substrate along a direction perpendicular to the substrate to actually get to the substrate.
With such an apparatus, however, atoms emitted from the target mostly and temporarily adhere to the filter disposed between the target and the substrate and eventually fall on the substrate as dust.